Enteric caliciviruses of pigs and cattle

N J KNOWLES AND I N CLARKE

GENERAL INTRODUCTION: CALICIVIRIDAE

Introduction and aetiology

A range of viruses has been associated with enteric disease in farm animals. Viruses resembling caliciviruses were first described in cattle in 1978^{1, 31} and in pigs in 1980.²³ Woode and Bridger (1978)³¹ in pioneering work examined diarrhoeic faecal samples from three independent UK cattle herds: viruses were found in faecal samples from a calf in each herd. Each sample contained small round viruses (SRVs) based on their appearance under the transmission electron microscope (TEM), but a number of other different viruses including rotaviruses (sample 1) and coronaviruses (sample 2) were also present. The presence of several viruses in calf diarrhoeic faecal samples is not uncommon but it does cause significant problems with establishing an aetiological link between the presence of the virus and the cause of the disease.

During the course of experiments in calves with the samples collected it was also realized that all three original faecal specimens harboured two morphologically distinct SRVs; in each case they contained both calici-like viruses and astroviruses. Attempts to grow the calici-like virus in cell culture were unsuccessful, and in order to separate the different SRVs, the calici-like virus in sample 1 (now known as Newbury agent 1, or NA1) was separated from the astrovirus by infecting a gnotobiotic calf with astrovirus and then by challenging the previously infected animal orally with a faecal filtrate containing the mixture of SRVs. Faeces from the challenged calf were used in oral serial passage of another four calves. Only the calici-like virus particles (NA1) were observed in the faeces of these calves. Further passage of sample 2 in gnotobiotic calves lost the astrovirus, leaving a small round virus which was identified as SRV2a as the sole cause of disease. It is now renamed Newbury agent 2 (NA2).

There were some subtle differences in the morphology of NA1 and NA2, but neither virus exhibited the clear morphological characteristics of the ‘classic’ caliciviruses seen in other animal species. It was later realized that the Newbury agents most closely resemble the human Norwalk virus.² In a separate study of diarrhoeal disease in calves, viruses similar to the human prototype Norwalk virus were observed in the faeces of new-born calves in Thuringia, Germany.¹² Other German workers also observed Norwalk-like viruses (NLVs) in the faeces of calves.⁹ Whilst all these observations were made in the late 1970s and 1980s it was not until the late 1990s that the first complete genome sequence was published for a German virus — the Jena virus (JV).¹⁸ This showed unequivocally that JV belonged to the Caliciviridae. Phylogenetic analysis indicated that JV is most closely related to the human NLVs. Jena virus has a similar genome organization to Norwalk virus, and the JV genome encodes three open reading frames (ORFs), a large ORF (ORF 1) that encodes the polyprotein, ORF 2 the capsid protein, and a 3’ terminal ORF (ORF 3) that encodes a small basic protein that has been speculated in other caliciviruses to have a structural role. Partial sequence of the 3’ terminal third of NA2 revealed a similar genome organization.⁵

The family Caliciviridae comprises four genera, Vesivirus, Lagovirus, Norovirus (Norwalk-like viruses) and Sapovirus (Sapporo-like viruses).¹⁰ The human Norwalk-like viruses (often referred to as Small Round Structured Viruses, or SRSVs) have been divided into two genotypes, I and II. The bovine enteric caliciviruses (BECVs) which have so far been characterized by nucleotide sequencing are JV,¹⁸ NA2⁵, NB and CV23-OH²⁴ and 14 sequences of a calicivirus amplified from faecal samples in the Netherlands.²⁷ With the exception of NB and CV23-OH, which appear to belong to a previously undescribed novel genus ²⁴ (Figure 57.1), all are closely related to each other and belong to the genus Norovirus. They are more closely related to the genotype I SRSVs than to those belonging to genotype II (Figure 57.1). Porcine (or swine) enteric caliciviruses also fall into two distinct groups, one belonging to the Norovirus genus^{25, 27} and the other to the Sapovirus genus.^{13, 27, 28} To distinguish more easily between the porcine enteric caliciviruses belonging to the two genera, they are referred to below as porcine enteric noroviruses (PENVs) and porcine enteric sapoviruses (PESVs).

Figure 57.1 Unrooted neighbour-joining tree based on a comparison of the complete capsid sequences of members of the four calicivirus genera, Vesivirus, Lagovirus, Norovirus and Sapovirus, and a newly described distinct bovine enteric calicivirus. (NB: Amino acid sequences were obtained from the EMBL/GenBank databases, aligned and the tree constructed using Clustal X.²⁶ The tree was drawn using TreeView.¹⁹) Abbreviations: BECV, bovine enteric calicivirus; CaCV, canine calicivirus; EBHSV, European brown hare syndrome virus; FCV, feline calicivirus; HuCV, human calicivirus; PCV, primate calicivirus; PENV, porcine enteric norovirus; PESV, porcine enteric sapovirus; RHDV, rabbit haemorrhagic disease virus; SMSV, San Miguel sealion virus; SRSV, small round structured virus; VESV, vesicular exanthema of swine virus

It is now clear that BECVs cause enteric disease in calves, the ensuing disease being of similar severity to that caused by...